1. Field of the Invention
The present disclosure related to a photoelectric conversion apparatus, a focus detection apparatus, an image pickup system, and a method of driving a photoelectric conversion apparatus.
2. Description of the Related Art
An image pickup system typically includes an auto focusing (AF) sensor that detects a focus. A recent AF sensor requires not only multipoint of the number of ranging points but also high precision focus detection. As a means to increase the number of ranging points, an area-type AF is employed in which a plurality of linear sensors that configures the ranging points is disposed in parallel and is connected with a common wiring in the line direction. As a circuit configuration of the area-type AF, Japanese Patent Application Laid-Open No. 9-200614 discloses a solid-state image pickup apparatus provided with a transfer system that transfers a signal output from a sensor cell unit to a memory cell unit. To enhance the focusing precision of AF by increasing an SN ratio of a sensor signal, Japanese Patent Application Laid-Open No. 9-200614 causes the sensor cell unit and the memory cell unit to have an inverting output function of a signal to remove fixed pattern noises of the sensor cell unit and the memory cell unit itself.
Further, it is known that the AF sensor detects the highest value and the lowest value of an accumulation signal and performs an operation of properly controlling an amplitude value of the accumulation signal (auto gain control: AGC) in order to realize a broad dynamic range. The precision of accumulation control relates to the AF precision. Therefore, by removing a fixed pattern noise of a sensor from the accumulation signal used in AGC, the AF precision can be enhanced. Japanese Patent Application Laid-Open No. 9-200614 removes the fixed pattern noises from the accumulation signal by holding the fixed pattern noises of the sensor cell unit and the transfer unit in the sensor cell unit.
However, like Japanese Patent Application Laid-Open No. 9-200614, if the fixed pattern noises are held in the sensor cell unit, there may be a problem when a detected capacity of a photocarrier is made small. This is because a switch means of the sensor cell unit in Japanese Patent Application Laid-Open No. 9-200614 becomes a new cause of noise generation. In a case where the switch means is configured from a MOS transistor, fluctuation of gate potential is propagated to a source and drain of the MOS transistor at the time of switch OFF (clock feed-through). Further, a carrier held under gate at the time of switch ON is moved to the source or drain at the time of switch OFF (charge injection). A voltage fluctuation amount of the source and drain at the time of switch OFF depends on the potential of the source and drain immediately before the switch OFF. That is, like Japanese Patent Application Laid-Open No. 9-200614, when the fixed pattern noises are held in a photodiode of the sensor cell unit through the switch means, variation corresponding to the fixed pattern noises appears in the source and drain potential among pixels. Then, the voltage fluctuation amount caused at the time of switch OFF varies among the pixels. Therefore, the fixed pattern noises held in the photodiode are different from fixed pattern noises that are intrinsically supposed to be removed. As a result, the fixed pattern noises cannot be sufficiently removed from the accumulation signal at the AGC, and therefore, the AGC precision and the AF precision are lowered.
In recent years, the AF sensor also requires high sensitivity with the requirement of the high sensitivity for an image pickup element. To enhance the high sensitivity of a sensor, the detected capacity is made small. However, from the above reason, there is a problem such that the voltage fluctuation at the time of switch OFF becomes large and the AGC precision is lowered, when the detected capacity is made small.